Modified steel rule die

ABSTRACT

The formation of plastic bodies from thermoplastic sheet using the hot platen technique and trimming in place in the mold, wherein the molded product is removed from the forming press while still attached to the sheet. A sheet having the molded product attached thereto but much more easily removed is formed by using a modified steel rule die. The die is modified by cutting small slots at a plurality of points in the outside edge of the blade so that at the point of slotting, the cutting edge of the die is narrower than the rest. When the die is pressed into the sheet following the molding step, a long crack forms around the periphery of the product until the point of a slot is reached and a small crack forms along the length of the slot leaving an attachment to the sheet between the long and the small cracks.

llnited States Patent [191 Clayton et al.

[ MODIFIED STEEL RULE DIE [75] Inventors: William J. Clayton, Fairport;Clair CJSmith, Holcombe; Earl K. Dewey, Canadaigua, all of NY.

[73] Assignee: Mobil Oil Corporation, New York,

22 Filed: Mar. 29, 1971 [21] Appl. No.: 129,232

Related U.S. Application Data [62] Division of Ser. No. 849,721, Aug.13, 1969,

abandoned. v

[52] US. Cl. 225/103, 83/695 [51] Int. Cl B26f 3/02 [58] Field ofSearc'h.....' 264/163; 83/652,

[56] 0 References Cited UNITED STATES PATENTS 3,520,220 7/1970 Acker83/695 X [11] EfidiAfiZ lane 26, 1973 Primary Examiner--Andrew R. JuhaszAssistant Examiner-Leon Gilden Attorney;- Oswald G. Hayes, Hastings S.Trigget a1.

[5 7] ABSTRACT The formation of plastic bodies from thermoplastic sheetusing the hot platen technique and trimming in place in the mold,wherein the molded product is removed from the forming press while stillattached to cutting edge of the die is narrower than the rest. When thedie is pressed into the sheet following the molding step, a long crackforms around the periphery of the product until the point of a slot isreached and a small crack forms along the length of the slot leaving anattachment to the sheet between the long and the small cracks.

1 Claim, 3 Drawing Figures MODIFIED STEEL RULE DIE This application is adivision of Ser. No. 849,721, filed Aug. 13, I969, now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention isdirected to the formation of plastic shapes from thermoplastic sheet bythe hot platen tech nique. It is more particularly concerned with amodified steel rule die for use in the process.

2. Description of the Prior Art As is well known to those familiar withthe art, in the hot platen process of thermoforming from thermoplasticsheet, the sheet is held against a hot platen by a pressure differentialand then forced into the mold cavity by a reverse pressure differential.As is described in U.S. Pat. No. 3,190,946, a steel rule die (thereincalled a dull'knife 84), around the mold periphery first holds the sheetduring molding and then is forced into the sheet to form a crease butdoes not sever the sheet. The sheet with molded product still attachedby a thin layer of plastic is moved in a nonlinear path so that theproduct breaks out of the sheet. The degree of attachment between themolded product and the sheet, however, is highly variable, attributablein part to variations in the thickness of the sheet. If the attachmentis not sufficiently secure, and only a single molded product is joltedout of the sheet prematurely the operation must be'stopped and anadjustment must be made before production can continue.,Also, if theattachment of a single molded product is too secure and the product doesnot break out when it is supposed to, again production must be stoppedfor adjustment. It has also been proposed in the aforedescribedprocedure, instead of creasing the sheet, to penetrate the sheet withthe steel rule die to an extent that the sheet is cracked (not cut)through. In order to retain attachment of product to the sheet, notchesare cut in the cutting edge of the die so that the crack is interruptedat intervals about the periphery and the solid attachments are retained.When the product is separated from the sheet, however, thereremain'sharp projections on the product that can cut fingers and tent tocut the film overwrap, necessitating repackaging.

SUMMARY OF THE INVENTION This invention provides, in the process offorming plastic bodies from thermoplastic sheet using the hot platentechnique and trimming in place in the mold with a steel rule die aroundthe periphery of the mold cavity'whereby the molded product is removedfrom edge and having a plurality of small slots at intervals v along thebeveled edge to form a narrower dull cutting edge at the slots.

DESCRIPTION OF THE DRAWING FIG. 1 is a view of a section of a modifiedsteel rule die of this invention showing the dull cutting edge and aslotted portion therein;

FIG. 2 is a sectional elevation view of the steel rule die taken at 2-2in FIG. 1; and

FIG. 3 is a view of a section of sheet scrap and molded product showinga portion of the cuts resulting from the modified steel rule die of thisinvention.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS This invention provides amodified steel rule die or dull knife for use in the hot platen processsuch as described in' U.S. Pat. No. 3,190,946, to which reference ismade. In that process, however, the sheet is creased by the knife and isnot cut through. In using the modified steel rule die, it is forced intothe sheet a distance to crack it through the sheet where the diecontacts the sheet. With the modified steel rule die most of theperiphery of the molded product is severed from the sheet, but there arepoints of positive attachment, as is hereinafter described.

Referring now to FIGS. 1 and 2, there is shown a section of a standardsteel rule die 1 that has been sharpened by beveling along one edge 2 toform a dull flat cutting edge 3. The width of the cutting edge 3 can bebetween 2 mils and 8 mils wide, but preferably between 4 mils and 6mils.

Then, at a plurality of points along the steel rule die, slots 4 are cutinto the beveled edge 2 to form a narrower cutting edge 5. A depth of aslittle as 2 mils on the surface of the cutting edge works reasonablewell. A slot depth of 5 mils out of a total distance of 6 mils is themost satisfactory. From a practical standpoint, however, a depth ofabout half that of the original edge is the most convenient.

The width or length of the slot can be from about 1/ l 6 inch to anylength desired. A length less than 1/16 inch would be likely notto becompletely reproducible and a very long slot would be mechanicallyawkward. In preferred practice a length of one-eighthone-fourth inch isused.

FIG. 3 shows a portion of a sheet 10 after the molding and cracking(cutting) have been effected. Section 10a is the smooth scrap portionand 11 is a section of molded product. As the steel rule die iscompressed into the sheet against the platen 14, cracking occurs whensufficient compression is attained. The crack that forms is very smoothand straight and it forms along about the center line of the cuttingedge. Thus, when the modified steel rule die of this invention is usedto crack through the sheet, a crack 112 forms along the center line ofthe wider cutting edge 3 until a slot is reached. At that point, anothercrack 13 forms along the center line of the narrower cutting edge 5. Theattachment remaining between the molded product and the sheet scrap isthe distance (l-2 mils) between the two offset crack lines. This gives asecure attachment which, by the nature of the way it is located, breaksoff easily. The resultant molded product, after being broken free, willhave a smooth edge with small nicks where the slot portion has beenpositioned. These nicks and the breakoff points contribute so little tothe roughness of the edge that it is very difficult to find, unless aperson knows where to look. Further, there are no sharp protrusions thatcan cut fingers and overwrap.

The modified steel rule' die of this invention can be used in any methodof forming plastic articles from thermoplastic sheets using deepdrawing-die-forming techniques. Thus, there can be formed containers,such as meat and produce trays; toys; no'velties; and similar articles.

1. A modified steel rule die adapted for use with a platen beveled onone edge to form a narrow, planar dull cutting edge parallel to theplane of said platen and having a plurality of small slots at intervalsalong the beveled edge intersecting the plane of said cutting edge toform a narrower dull cutting edge at the slots.

1. A modified steel rule die adapted for use with a platen beveled onone edge to form a narrow, planar dull cutting edge parallel to theplane of said platen and having a plurality of small slots at intervalsalong the beveled edge intersecting the plane of said cutting edge toform a narrower dull cutting edge at the slots.